As one of devices for implementing a high-efficiency linear amplifier, there is a high frequency amplification circuit which employs an amplifier using linear amplification with nonlinear components (LINC).
FIG. 1 is a diagram illustrating an example of the amplifier using LINC. In the amplifier 10 using LINC, a LINC signal generator 2 separates an input modulated signal Sin(t) into a pair of phase-modulated signals Sc1(t) and Sc2(t) having a phase difference corresponding to an amplitude of the input modulated signal Sin(t) and generates the pair of the phase-modulated signals Sc1t) and Sc2(2). For example, the input modulated signal Sin(t) is a modulated signal subjected to an amplitude modulation and a phase modulation (angle modulation), and the pair of phase-modulated signals Sc1(t) and Sc2(t) are constant amplitude phase-modulated signals forming a constant envelope. The input modulated signal Sin(t) and the pair of phase-modulated signals Sc1(t) and Sc2(t) here may be all baseband signals, or may be all intermediate frequency (IF) signals. The LINC signal generator 2 outputs the pair of phase-modulated signals Sc1(t) and Sc2(t) as a digital signal.
Here, the signals Sin(t), Sc1(t) and Sc2(t) are expressed, for example, as follows.
            Sin      ⁡              (        t        )              =                            a          ⁡                      (            t            )                          ·        cos            ⁢                          ⁢              θ        ⁡                  (          t          )                                Sc      ⁢                          ⁢      1      ⁢              (        t        )              =                  a        max            ·              cos        ⁡                  (                                    θ              ⁡                              (                t                )                                      +                          ψ              ⁡                              (                t                )                                              )                                Sc      ⁢                          ⁢      2      ⁢              (        t        )              =                  a        max            ·              cos        ⁡                  (                                    θ              ⁡                              (                t                )                                      -                          ψ              ⁡                              (                t                )                                              )                                ψ      ⁡              (        t        )              =                  cos                  -          1                    ⁡              (                              a            ⁡                          (              t              )                                            2            ·                          a              max                                      )            
where a(t) indicates an amplitude component of the input modulated signal Sin(t), and θ(t) indicates a phase component of the input modulated signal Sin(t). A phase modulation is performed so as to generate a phase difference 2×ψ(t) corresponding to the amplitude a(t). In addition, amax indicates the maximum value of the amplitude a(t) and is a constant. The signals Sc1(t) and Sc2(t) are constant envelope signals. That is to say, the amplitudes of the signals Sc1(t) and Sc2(t) are constant.
The signal Sc1(t) that is one of the pair of phase-modulated signals output from the LINC signal generator 2 is converted from a digital signal to an analog signal by a digital to analog converter (DAC) 16. In addition, the converted analog signal passes through a low-pass filter 18, and thereby a component corresponding to a frequency band of the signal Sc1(t) that is one of the pair of phase-modulated signals is extracted and other frequency components are suppressed. Similarly, the signal Sc2(t) that is the other of the pair of phase-modulated signals is converted from a digital signal to an analog signal by a DAC 36. In addition, the converted analog signal passes through a low-pass filter 38, and thereby a component corresponding to a frequency band of the signal Sc2(t) that is the other of the pair of phase-modulated signals is extracted and other frequency components are suppressed.
In the amplifier 10 using LINC, a quadrature modulator 20 performs a quadrature modulation on the signal Sc1(t) that is one of the pair of phase-modulated signals and has passed the low-pass filter 18. A frequency converter 22 generates and outputs a signal S1(t) that is a radio frequency (RF) signal and one of a pair of high frequency signals, by using a high frequency signal (oscillation signal) output from an oscillator (not illustrated). Similarly, a quadrature modulator 40 performs a quadrature modulation on the signal Sc2(t) that is the other of the pair of phase-modulated signals and has passed the low-pass filter 38. A frequency converter 42 generates and outputs a signal S2(t) that is an RF signal and the other of the pair of high frequency signals, by using a high frequency signal (oscillation signal) output from an oscillator (not illustrated).
The high frequency signal S1(t) and the high frequency signal S2(t) are expressed as follows. Here, a radio frequency (a frequency of the oscillator) is indicated by fc.S1(t)=amax·cos(2π·fc·t+θ(t)+ψ(t))S2(t)=amax·cos(2π·fc·t+θ(t)−ψ(t))
A pair of amplifiers include two amplifiers 24 and 44 which are provided in parallel. Gain and phase characteristics of the two amplifiers 24 and 44 are substantially the same. The amplifiers 24 and 44 amplify the high frequency signals output from the frequency converters 22 and 42, respectively.
A combiner 52 combines the pair of high frequency signals amplified by the pair of amplifiers 24 and 44 and outputs the combined signal as a high frequency signal Sout(t). The signal Sout(t) output from the combiner 52 is expressed as follows when the gain of the pair of amplifiers 24 and 44 is indicated by G.
                              S          ⁢                                          ⁢                      out            ⁡                          (              t              )                                      =                ⁢                              G            ·                          a              max                        ·                          cos              ⁡                              (                                                      2                    ⁢                                          π                      ·                      fc                      ·                      t                                                        +                                      θ                    ⁡                                          (                      t                      )                                                        +                                      ψ                    ⁡                                          (                      t                      )                                                        +                  ϕ                                )                                              +                                                ⁢                  G          ·                      a            max                    ·                      cos            ⁡                          (                                                2                  ⁢                                      π                    ·                    fc                    ·                    t                                                  +                                  θ                  ⁡                                      (                    t                    )                                                  -                                  ψ                  ⁡                                      (                    t                    )                                                  +                ϕ                            )                                                              =                ⁢                  2          ⁢                                          ⁢                      G            ·                          a              max                        ·                          cos              ⁡                              (                                                      2                    ⁢                                          π                      ·                      fc                      ·                      t                                                        +                                      θ                    ⁡                                          (                      t                      )                                                        +                  ϕ                                )                                              ⁢                      cos            ⁡                          (                              ψ                ⁡                                  (                  t                  )                                            )                                                              =                ⁢                  2          ⁢                                          ⁢                      G            ·                          a              ⁡                              (                t                )                                      ·                          cos              ⁡                              (                                                      2                    ⁢                                          π                      ·                      fc                      ·                      t                                                        +                                      θ                    ⁡                                          (                      t                      )                                                        +                  ϕ                                )                                                        
Here, φ is a passing phase of the pair of high frequency signals S1(t) and S2(t).
Japanese Examined Patent Application Publication No. 08-31886, Japanese Laid-open Patent Publication No. 05-37263, Japanese Laid-open Patent Publication No. 2004-343665, Japanese Laid-open Patent Publication No. 2008-167289, Japanese National Publication of International Patent Application No. 2002-506309, Japanese Laid-open Patent Publication No. 2000-349575, and Japanese Laid-open Patent Publication No. 2011-193472 are examples of the related art.